From my experience the only people riding BMX wearing a mouth guard are Yanks, who have a thing about pearly whites and smiles that glisten, any way that’s my tuppence worth,

If you grind your teeth without one in your still gonna do it regardless, the only difference is the guard will delaminate,
From my own point of view the less obstruction in your mouth the better, maybe fine in a foam pit or at a park but out on the trail on your loan some, I wouldnt.